Rechargeable electrochemical cells may be used as direct replacements for primary AA, C, and D cells in numerous consumer devices such as calculators, portable radios, and flashlights. Often, rechargeable cells are integrated into a sealed power pack that is designed to interface with a specific device. In sealed power packs, weight and portability are important considerations. Long operating life and maintenance free operation are desirable characteristics for all rechargeable cells.
A rechargeable cell is ideally suited to serve as a portable power source due to its small size, light weight, high power capacity and long operating life. A rechargeable cell may operate as an "install and forget" power source. With the exception of periodic charging, such a rechargeable cell typically performs without attention and rarely becomes the limiting factor in the life of the device it powers.
Rechargeable cells using a hydrogen storage negative electrode are known in the art. See, for example, U.S. Pat. No. 4,551,400 (hereinafter "the 400 patent") for HYDROGEN STORAGE MATERIALS AND METHODS OF SIZING AND PREPARING THE SAME FOR ELECTROCHEMICAL APPLICATIONS, the disclosure of which is incorporated by reference. (Cells using rechargeable hydrogen storage negative electrodes are hereinafter referred to as "hydrogen storage cells.") Hydrogen storage cells operate in a different manner from lead-acid, nickel-cadmium, or other prior art battery systems. The metal hydride negative electrode used in hydrogen storage cells is capable of the reversible electrochemical storage of hydrogen. In hydrogen storage cells, the positive electrode is typically formed of a nickel hydroxide material. A suitable separator, spacer, or membrane may be positioned between the negative and positive electrodes.
Hydrogen storage cells operate with a nickel hydroxide positive electrode; a hydrogen storage alloy negative electrode; and a non-woven, felted, nylon or polypropylene separator. The electrolyte is generally 20 to 45 weight percent potassium hydroxide.
Hydrogen storage cells offer important advantages over conventional rechargeable cells. Hydrogen storage cells have significantly higher specific charge capacities (both in terms of ampere hours per unit mass and ampere hours per unit volume) than do cells that use lead or cadmium negative electrodes. As a result of the higher specific charge capacities, a higher energy density (in terms of watt hours per unit mass or watt hour per unit volume) is possible with a hydrogen storage cell than is possible with prior art systems such as the lead acid and NiCd. Thus, hydrogen storage cells are particularly suitable for many commercial applications.
The operation of a hydrogen storage cell produces hydroxyl ions, and may also produce gases under certain circumstances. As a result, the internal cell pressures may vary substantially during operation of a hydrogen storage cell. Because of this, hydrogen storage cells are typically produced as either sealed cells or vented cells. During normal operation, a sealed cell does not permit the venting of gas to the atmosphere. In contrast, a vented cell will release excess pressure by venting gas as part of its normal operation. As a result of this difference, the vent assemblies used in sealed and vented cells are quite different from one another, and the amounts of electrolyte in the cell container relative to the electrode geometry differ significantly.
Sealed cells are manufactured predominantly in cylindrical and rectangular configurations. Sealed cells are usually designed to operate in a starved electrolyte configuration. The cell enclosure for a sealed cell is normally metallic and designed for operation at pressures up to about 100 pounds per square inch absolute or even higher. Sealed cells are characterized by the substantial absence of any required maintenance.
A variation of sealed cells are cells containing "one time only" venting mechanisms, for example, a rupturable diaphragm and blade apparatus. As internal pressure increases, the blade is forced against the diaphragm. As the pressure further increases to the pressure limits of the particular cell configuration, the blade punctures the diaphragm, allowing excess gases to escape through the ruptured diaphragm without catastrophic cell failure.
Vented hydrogen storage cells, which have a nickel hydroxide positive electrode, and a hydrogen storage alloy negative electrode, typically employ a woven or non-woven separator. Vented cells differ most significantly from sealed cells in that they operate in a flooded condition. A "flooded condition", as used herein, refers to a cell in which the electrodes are completely immersed in electrolyte. Such cells are sometimes referred to as "flooded cells." A vented cell is further distinguished from a sealed cell in that it is designed for normal operating pressures of only up to about 10 pounds per square inch, after which excess pressures are relieved by a vent mechanism.
As discussed above, operation of a hydrogen storage cell produces hydroxide ions and can produce various gases. The amount of gases generated depends on the amount of electrolyte, the operating temperature, as well as variations in components, chemical concentrations, and manufacturing techniques.
Quality control review of all types of rechargeable cells generally involves measuring the pressures developed in finished cells during charge/discharge cycling. There are several methods of pressure measurement known in prior art. One involves the use of a strain gauge. Once amount of strain induced by the pressures developed in a cyclindrical can are very small. Unless very sensitive strain gauges are used, the measurements will be erroneous. A "one time only" method of instantaneous pressure measurement involves puncturing the can with a sharp "nail like" device connected to a gauge. Once measured, the cells cannot be resealed. With this method it is impossible to make multiple pressure measurements. The third method involves putting a hole through the lid or side and connecting a transducer to it. The disadvantage of this is that even a small amount of pressure in the cell serves to expel electrolyte through the drilled hole, thereby reducing the quantity of the electrolyte. Since the quantity of electrolyte lost is unpredictable and since pressure is dependent upon the quantity of electrolyte, pressures measured in this manner cannot be meaningfully interpreted. Additionally, meaningful data requires the measurement of pressure over hundreds of charge/recharge cycles. The pressure measurement techniques of the prior art required a new cell to be sacrificed for each measured data point.
Thus, not only were hundreds of cells sacrificed for each set of measurements, but variations from cell to cell had to be taken into consideration in interpreting the results.
The hydrogen storage cells described in the '400 patent, require a heat treatment process for activation. Pressure tends to build up within the cell during this activation process. When a hole is made in the can in order to make a pressure measurement, this activation process greatly contributes to the loss of electrolyte. Even if the hole is formed prior to charge/discharge cycling, the activation process nonetheless initiates a pressure rise and a corresponding loss of electrolyte, which results in a reduction of the internal pressure. Tape cannot be used to cover the hole as the electrolyte tends to "creep" through the hole and destroy the adhesive on the tape. Even in those instances in which the tape survives the initial activation, the pressure rise causes electrolyte leakage when the tape is punctured after heat treatment. Attempts to secure the tape to cover the hole internally have been unsuccessful because the tape interferes with the process of inserting the core into the can; and the electrolyte attacks and eventually releases the adhesive.
A need exists to more efficiently measure the internal pressure in all types of rechargeable cells, particularly metal hydride cells, during normal and worst case operation. Such a pressure measurement must be non-destructive and compatible with continuous charge/discharge cycling of the cell so that the pressures resulting from actual use of the cell can be determined without loss of electrolyte.